Base class for SLXNA

With the latest version of Windows Phone 7.5 Mango, there the possibility to mix Silverlight and XNA in one application.
This is very usefull to create buttons, pages,... while keeping the performace of XNA for the actual game.

The download for XNA WP7 includes very usefull base classes, but it's not useable for a SLXNA game because it inherits from the
Gamepage.